1. Field of the Invention:
This invention relates to electronic still cameras having a solid state image pickup element comprised of photosensitive elements, a vertical transfer CCD and a horizontal transfer CCD and a read drive circuit for the solid state image pickup element.
2. Description of the Related Art:
FIG. 1 is a diagram illustrating the outline of the structure of the image pickup element suited to the electronic still camera in the prior art and this invention.
In the figure, 1 is a full-frame transfer type CCD having photosensitive cells arrayed in the matrix form and also serving as a plurality of vertical transfer CCDs. Incident light impinging on a light receiving part 13 of the CCD 1 is converted to electric charge in the photosensitive cell and accumulated. When a vertical transfer pulse .phi..sub.P for vertically transferring the electric charge of the CCD is applied to the electrode 14, the accumulated electric charge is transferred to the horizontal transfer CCD 15 by one row, and further when a horizontal transfer pulse .phi..sub.S is applied to a horizontal transfer electrode 16, the electric charge transferred to the horizontal transfer CCD 15 is transferred horizontally, and converted to a voltage by an output amplifier 17. Also, 18 is a clear drain for draining out the unnecessary electric charge, and provided adjacent the CCD 15 through a gate. Also, 201 is a clear gate for controlling the potential barrier of the above-described gate between the horizontal CCD 15 and the clear drain 18. The pulse to be applied to this clear gate 201 is .phi..sub.CLR.
FIG. 2 is a diagram of the details of the output amplifier 17 for converting the electric charge of the CCD 1 to the voltage. 16a is a transfer gate; 20 is a MOS transistor; 21 is a reset drain; V.sub.DD is the drain voltage of the MOS transistor 20; FD is a floating diffusion (FD) layer; C.sub.FD is the coupling capacitance of this FD and the MOS transistor 20.
In FIG. 2, in the normal reading operation, letting the electric charge injected into the FD be denoted by .DELTA.Q, the detection signal .DELTA.V at the gate of the amplifier comprising the MOS transistor 20 becomes EQU .DELTA.V=.DELTA.Q/C.sub.FD
Here, when the horizontal transfer pulse .phi..sub.S is applied to a reset gate which is one of the transfer gates 16a, the electric charge .DELTA.Q is discharged to the reset drain 21, so that the FD is reset to the drain voltage V.sub.DD, and then the next electric charge is injected.
FIG. 3 is a timing chart of driving of the CCD 1 for the conventional electronic still camera.
In FIG. 3, the electronic camera has all electrical power sources applied in a state that the trigger output is high (H). A space from a time t.sub.0 to t.sub.1 is a clear period, where a high (H) voltage is applied to the clear gate 201, the potential barrier between horizontal transfer CCD 15 and the clear drain 18 is lowered, and the electric charges accumulated on the light receiving part 13 and the horizontal transfer CCD 15 are discharged to the clear drain 18. During an interval from a time t.sub.2 to t.sub.3, the shutter opens, and electric charge is accumulated on the light receiving part 13. After a time t.sub.4, a read period follows where the accumulated electric charge is vertically transferred by a vertical transfer pulse .phi..sub.P, and horizontally transferred by a horizontal transfer pulse .phi..sub.S, being read out at a high speed. For note, the clear gate 201 is made low (L) voltage so that the potential barrier between the horizontal CCD 15 and the clear drain 18 is heightened. The output wave forms of this time are shown in FIG. 4. For note, A, B, C are respectively the output wave forms of the image pickup element output, before the clamping and after the clamping.
As shown in FIG. 4, in the output wave form A, because, after the clear, the horizontal transfer pulse .phi..sub.S does not enter, the voltage gradually lowers.
This is, if with such a horizontal transfer pulse .phi..sub.S as in FIG. 3, no horizontal transfer pulse .phi..sub.S is not applied after the clear until the read start, so that the FD of FIG. 2 is not reset to the drain voltage V.sub.DD, and the electric charge is discharged from the C.sub.FD with gradual decrease of the output voltage. Therefore, like as the output wave form B of FIG. 4, the change of the potential of the time of a read start becomes so large that even if clamping is applied to the image pickup element output, the response of the clamping circuit cannot follow up. Like the output wave form C of FIG. 4, it takes a long time until the potential becomes constant. Also, the dark current collected in the horizontal transfer CCD 15 also is not reset. Therefore, there was an occasion that that dark current was outputted.